There is a technique of diagnosing a malfunction in a capacitor such as a bypass capacitor provided between power supply terminals of a semiconductor device. The technique of diagnosing a malfunction in a capacitor is, for example, described in JP 2010-259178 A and JP H10-71926 A.
Generally, there is a diagnosis method performed by using an external power supply and a POR (Power On Reset) circuit in a microcontroller in a semiconductor device. In such a method, first, an internal circuit of the microcontroller is stopped, and voltage of the external power supply is dropped for a moment. In a case where a bypass capacitor functions normally, since charges are accumulated in the bypass capacitor, even when the power supply voltage stops for a moment, the internal voltage of the microcontroller does not reduce below a lower limit voltage.
On the other hand, in a case where the bypass capacitor has an abnormality, since no charge is accumulated in the bypass capacitor, the internal voltage of the microcontroller will reduce below the lower limit voltage. When the internal voltage of the microcontroller reduces below the lower limit voltage, the POR circuit in the microcontroller operates and generates a reset signal to set the internal circuit to a reset state. In such a manner, the malfunction diagnosis of the bypass capacitor can be executed by the control of the external power supply and the POR circuit in the semiconductor device.
In the above-described technique, however, the malfunction diagnosis is executed on the basis of the presence or absence of the reset signal to the internal circuit. Thus, the malfunction diagnosis can be executed only in a state where the microcontroller is stopped.
In recent years, from the viewpoint of functional safety, it is required to detect a malfunction that exerts a critical influence on a system such as a power supply and a clock in a real-time manner. In the conventional technique in which the microcontroller needs to be stopped, it is difficult to detect a malfunction of the bypass capacitor in a real-time manner.